[ENH] remove electrodes from electrodeArray

[DylanLin29]

Description

• Contributes a method remove_electrode to the ElectrodeArray class (see issue Add a way to remove electrodes from an ElectrodeArray #147)
• Gets rid of add_electrodes of the ElectrodeArray class, because when electrodes are passed in a dictionary, there is no way to preserve the originally intended order (keys will automatically be sorted). Thus it's safer to call add_electrode in a for loop.

Checklist

• Docstrings for all functions
• Unit tests
• Clean style in the spirit of PEP8